' 


( 


A STUDY  OF  THE  GAS  EVOLVED 
BY  HEATING  SILVER  BROMATE 

BY 

william  Mclennan  Morgan 


thesis 


For  the  Degree  of  Bachelor  of  Science 


IN 

CHEMISTRY 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 


UNIVERSITY  OF  ILLINOIS 


1921 


UNIVERSITY  OF  ILLINOIS 


M8JL28* i9ri.. 

THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

Willi am  M c L ennan  M o r g an 

ENTITLED A..  ...tM...Saa...E)y.Dly.ad...by...Ee.a.tlng...S.i.l.v.e.r. 

Bromate 

IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 
DEGREE  OF 


Instructor  in  Charge 


Approved  : 


HEAD  OF  DEPARTMENT  OF 


’ 


\t)l\ 

N\^ 


A 

ACKNOWLEDGEMENT 
The  writer  wishes,  at  this 
time,  to  express  his  appreciation 
for  the  invaluable  assistance  of 
Doctor  J.  H.  Reedy,  who  suggested 
the  problem  and  under  whose  able 
direction  this  research  was  done. 


TABLE  OF  CONTENTS 


I  Introduction 

II  Experimental 

1.  Material  Used 

(a)  Preparation  of  the  Silver  Bromate 

(b)  Determination  of  purity 

2 Evolution  of  gas  upon  heating 

(a)  Methods  and  Apparatus 

(b)  Analysis  of  gas  samples 

(c)  Spectrum  of  Lhe  gas 

(d)  Analysis  of  residue 

(e)  Adsorptive  power  of  ignited  Silver  Bromate 

3 Properties  of  Silver  Bromate 

(a)  Behavior  towards  light 

(b)  Decomposition  upon  heating 

(c)  Existence  of  other  Silver  Oxy-bromine  Compounds 

4 Silver  Bromate  as  a standard  in  Iodimetry 
III  Summary 


TV  HI  hT  1 r->Vvw 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/studyofgasevolveOOmorg 


LIST  OF  PLATES 


Pag© 


I  Apparatus  Used  to  Collect  Gas  Evolved 

Upon  Heating  Silver  Bromate  5 

II  Apparatus  Used  for  Nitrogen  Determination  8 

it 

III  Plucker  Tube  Apparatus  Used  for  Spectrum 

Analysis  11 


1 

A STUDY  OF  THE  GAS  EVOLVED  BY  HEATING  SILVER  BROMATE 

I INTRODUCTION 

The  object  of  this  thesis  is  to  study,  in  general,  the 
decomposition  of  silver  bromate,  but  more  especially  to  study 
the  gas  evolved  by  silver  bromate  when  heated. 

In  the  work  on  the  transition  point  of  silver  bromate, 
which  was  carried  out  by  S.  J.  Gould  in  the  Chemical  Laboratory 
at  the  University  of  Illinois,  it  was  noticed  that  a gas  was 
evolved  upon  heating  silver  bromate  in  contact  with  a high- 
boiling  paraffin  oil. . The  evolution  of  this  gas  became  notice- 
able slightly  above  100  degrees  C.  Very  naturally,  it  was  at 
first  assumed  to  be  carbon  dioxide,  formed  by  the  oxidation  of 
the  oil.  At  the  same  time  there  was  a darkening  of  the  oil, 
suggesting  the  formation  of  asphaltic  substances  and  a blackening 
of  the  silver  salt.  Upon  removing  samples  of  the  silver  bromate 
residues,  washing  thoroughly  with  ether,  and  then  with  ice  water, 
using  small  amounts  only,  no  sensible  change  in  the  composition 
of  the  salt  could  be  detected.  This  led  to  an  attempt  to 
identify  the  gas.  The  test  for  carbon  dioxide  was  negative,  and 
as  only  a very  small  amount  of  the  gas  was  available  at  the  time, 
further  identification  of  the  gas  was  not  attempted. 

The  study  of  this  phenomena  has  been  resumed  and  extended 
so  as  to  include  the  general  chemical  behavior  of  silver  brom- 
ate. 

There  is  practically  nothing  in  the  available  chemical 


2 

literature  which  deals  with  this  problem,  hence  the  work  has 
been  more  interesting  because  of  tne  ract  that  the  problem  is 
almost  an  entirely  new  one. 

II  EXPERIMENTAL 

(D 

Preparation  of  the  Silver  Bromate 

In  the  present  work,  the  silver  oromate  was  prepared  from 
equivalent  amounts  oi  silver  nitrate  ana  potassium  uromate  free 
from  potassium  oromide.  The  silver  salt  was  covered  with  water, 
waxen  was  brought  to  an  incipient  ooil;  the  potassium  bromate 
was  added  in  tne  form  of  a hot  solution.  The  mixture  was 
digested  with  frequent  stirrings  for  two  to  three  hours,  ana 
then  thoroughly  cooled  in  ice  water.  The  precipitated  silver 
bromate  was  filtered  out,  and  the  digestion  process  repeated 
twice.  The  final  crop  of  crystals  was  dried  as  much  as  possible 
on  a suction  filter  and  then  transferred  to  a watch  glass  and 
heated  to  140  degrees  C.  for  an  hour  in  an  electric  drying  oven. 
The  product  was  pure  white,  and  has  been  preserved  for  several 
months  over  anhydrous  calcium  chloride  in  a desiccator  with  no 
darkening  effect  whatever.  An  interesting  thing  might  be 
mentioned  in  this  connection: 

The  decomposition  of  silver  bromate  at  high  temperatures 
is  hastened  by  contact  with  certain  substances  that  might  be 
considered  to  be  inert.  A quantity  of  silver  bromate  crystals 
were  heated  on  a porous  porcelain  drying  plate  in  an  electric 
oven  for  about  one  hour,  the  maximum  temperature  being  131  deg.C. 


a-— il 


t 


: ... 

. j 


. . 


- 


• ' . • . c/isS 

4 


. 

; 

. • ■ V 

. ; 

. 

. . 


• ■ ■ ; 

• • • , 


3 


The  part  in  contact  with  the  plate  was  quite  yellow,  indicating 
the  formation  of  silver  bromide.  This  seems  to  have  been  due  to 
a catalytic  effect  of  the  clay  on  the  decomposition  of  silver 
bromate,  rather  than  an  actual  reduction,  and  it  is  important 
to  note  that  in  reduction  actions  upon  silver  bromate  as  upon 
all  silver  salts,  the  product  is  usually  black.  When  the  drying 
was  carried  out  on  a glass  plate  no  decomposition  occurs  as  when 
the  clay  plate  was  used. 

Determination  of  the  purity  of  Silver  Bromate 

The  purity  of  the  recrystallized  silver  bromate  was  deter- 
mined by  converting  it  into  silver  bromide  by  means  of  hydro - 
bromic  acid,  and  weighing  the  residue  after  evaporation.  The 
reaction: 

AgBrO^-t  6HBr  — ► AgBr  + 3H20  + 3Br2 

The  hydrobromic  acid  was  carefully  redistilled  so  as  to 

remove  all  non-volatile  matter.  The  record  of  this  work  is: 

(0  (2) 


wt. 

of 

dish 

silver  bromate 

33*2111 

gms 

38.8524 

tl 

11 

tl 

32.6989 

38.3369 

u 

1! 

it  it 

.5122 

• 5155 

Wt'. 

of 

dish 

silver  bromate 

33. 1068 

guts 

38.7475 

n 

11 

n 

32 . 6989 

38.3369 

n 

II 

: ” " (exp) 

.4079 

.4106 

ii 

11 

" " Uet) 

.4079 

.4106 

Purity 

U)Q/o 

100^ 

t ' 


. 


. 

■ 


4 


Analysis  of  the  samples  shows  that  the  usual  purity  of 

silver  bromate  prepared  In  this  way  ranges  from  99*6  to  99.8$, 

but  that  on  prolonged  heating  for  several  hours  at  130  to  135 

degrees  C.  a purity  of  100$  was  obtainable.  The  deviation  from 

absolute  purity  might  probably  be  due  to  the  presence  of  a small 

amount  of  silver  perbromate,  but  the  most  likely  assumption  is 

(2) 

that  the  foreign  substance  is  water  or  air.  Stas  has  reported 
that  silver  bromate  of  100$  purity  could  not  be  prepared,  owing 
to  the  fact  that  the  last  traces  of  water  could  not  be  driven 
off  below  the  decomposition  point  of  the  salt.  This  supports 
the  assumption  that  the  impurity  is  water.  On  the  other  hand, 
however,  it  was  later  discovered  in  this  research  that  a one 
gram  sample  of  silver  bromate  adsorbs  two  cubic  centimeters  of 
air  which  would  amount  to  an  impurity  of  about  .3$  just  as  found 
above.  It  therefore  seems  to  be  a very  reasonable  assumption 
that  the  impurity  is  adsorbed  air. 

Evolution  of  G-as  upon  Heating 
Methods  and  Apparatus 

In  order  to  obtain  samples  of  the  gas  which  Gould  noticed 
was  formed  upon  heating  silver  bromate  in  contact  with  high- 
boiling  paraffin  oils,  apparatus  of  the  form  shown  in  Figure  I 
was  devised.  Two-gram  samples  of  silver  bromate  were  introduced 
into  the  bulbs.  In  one  tube  the  salt  was  covered  with  fresh 
conductivity  water,  so  as  to  exclude  as  much  as  possible  the 
presence  of  dissolved  air.  A high-boiling  paraffin  oil  was  used 


fbjRUjvj  Vn?  jo  (ouicj  (jis  tvoivu  VjM  Hupucj  ^ivtt  Uimujt  * 5 


in  the  second  apparatus.  To  keep  out  air,  an  atmosphere  of 
hydrogen  was  maintained  in  the  set-up.  The  two  tubes  were 
arranged  in  series,  and  the  bulbs  immersed  in  the  same  bath  of 
cottonseed  oil  which  was  kept  at  a temperature  of  about  110 
degrees  G.  by  means  of  a carbon  incandescent  electric  light. 
The  gas  was  evolved  more  rapidly  in  the  water-filled  tube  than 


. 


6 


r 

in  the  oil  filled  one,  the  ratio  being  about  four  to  one.  This 
was  possibly  due  to  greater  viscosity  and  to  the  fact  that  the 
oil  may  have  united  chemically  with  some  of  the  oxygen  liber- 
ated, forming  asphaltic  substances.  The  volume  of  the  gas 
collected  in  the  apparatus  containing  water  was  8.4  cubic  cent- 
imeters in  two  to  three  weeks. 

Analysis  of  G-as  Samples 

The  gas,  which  was  evolved  when  the  silver  bromate  was 
heated  under  water,  was  analyzed  in  the  following  manner: 

The  collected  gas  was  treated  with  potassium  hydroxide  to 
remove  any  carbondioxide  that  might  possibly  be  present,  then 
with  alkaline  pyrogallol  to  remove  the  oxygen.  As  these  two 
absorbents  remove  carbon  dioxide  and  oxygen  quantitatively, 
the  contraction  in  volume  enables  the  analyst  to  calculate  the 
percent  of  the  various  constituents  present.  The  hydrogen  was 
removed  by  sparking  with  a known  amount  of  pure  oxygen  in  a 
eudiometer.  The  excess  oxygen  was  later  removed  by  absorption 
in  alkaline  pyrogallol.  The  fact  that  there  was  no  contraction 
in  volume  after  the  sparking  indicates  that  there  was  no 
hydrogen  present  in  the  gas.  As  is  the  usual  procedure  in  gas 
analysis  the  nitrogen  was  determined  by  difference.  The  record 
of  the  analysis  of  two  samples  of  the  gas  is  as  follows: 


. 


7 


Sample  I 


Original  volume 

8.4  cc 

After  KOH 

8.4 

co2 

none 

After  alkaline  pynogallol 

6 . 9 

°2 

1.5  cc 

17.90$ 

Oxygen  added  (pure) 

3-3 

— 

Total  volume 

10.2 

Sparked  for  hydrogen 

10.2 

h2 

none 

Nitrogen  by  difference 

h2 

6.9  cc 

82.10$ 

Sample  II 

Original  volume 

5*9  cc 

After  KOH 

5-9 

co2 

none 

After  alkaline  pyrogallol 

4.9 

o2 

1 .0  cc 

16.95$ 

Oxygen  added  (pure) 

2.3 

Total  volume 

7.2 

Sparked  for  hydrogen 

7.2 

none 

Nitrogen  Dy  difference 

n2 

4.9  cc 

83.05$ 

These  two  analyses  agree  very  well  for  experimental  work, 
hence  all  assumptions  in  the  remainder  of  this  paper  will  he 
based  upon  them. 

The  test  for  nitrogen  was  made  qualitatively.  The 
apparatus  as  shown  in  Figure  II  was  evacuated  and  then  filled 
with  hydrogen.  In  the  interior  of  the  evacuated  tube  were 
several  pieces  of  magnesium  ribbon.  The  gas  upon  which  the  test 
for  nitrogen  was  to  oe  made,  was  placed  in  the  tube  with  the 


4 

. 

• 

. 

* 

. 

. 

. * . c 

. 


. 

. 


8 


Jlmwm  fbtp  fins.  Hnutt*  hTtinutiniwi 


n 


1 


magnesium  ribbon,  the  stopcock  closed  and  the  tube  heated  until 
the  nitrogen  present  combined  with  the  magnesium. 

3Mg  -1-  Np ► Mg, Up 

After  the  reaction  was  completed,  the  stopcock  was  opened 
and  the  water  from  the  large  dish  allowed  to  enter  the  bulb. 

The  water  was  allowed  to  react  with  the  magnesium  nitride,  and 
the  resulting  ammonia  gas  absorbed  in  hydrochloric  acid.  The 

I 

ammonium  chloride  formed  was  neutralized  with  sodium  hydroxide 
and  the  solution  boiled.  The  odor  of  ammonia  given  off  during 

! i 

I { 

-■  - si 


, 


■ 


9 


the  boiling,  and  the  fact  that  the  fumes  evolved  turned  blue 
litmus  red,  indicated  conclusively  that  the  original  gas 
contained  nitrogen. 

Mg3N2+  6H20  3Mg(0H)2+  2NH3 

2NH,+  2HC1 2NH.C1 

3 4 

NH.C1  + NaOH  - NaCl  + H.O  + NH_ 

4 2 3 

Since  all  halogen  compounds  would  have  been  absorbed  in  the 
water  over  which  the  gas  was  collected,  it  is  certain  that  none 
of  them  were  present  in  the  gas  as  analyzed,  and  as  nitrogen  is 
always  determined  by  difference  in  gas  analyses  it  may  be 
considered  that  nitrogen  is  present  in  this  gas  to  the  extent  of 
82.10^.  This  would  indicate  that  the  gas  evolved  Dy  silver 
Dromate  when  heated  is  adsorbed  air.  Since  the  amounts  of 
oxygen  ana  nitrogen  in  the  air  are  21 % and  19%  respectively,  it 
is  seen  from  the  above  data  that  silver  bromate  is  a better 
absorbent  for  nitrogen  than  for  oxygen. 

Analysis  of  the  gas  samples  obtained  from  the  silver  bromate 
under  oil,  while  they  were  all  smaller  in  quantity,  gave  the 
same  analysis  of  the  gas,  showing  that  the  gas  came  from  the 
silver  salt  and  not  from  the  liquid  with  which  it  was  covered. 


. 


- 


. 


. 


. 


* 


* 


Spectrum  of  the  gas 


10 


In  making  a spectrum  analysis  of  the  gas,  a sample  of 
approximately  six  cubic  centimeters  was  used.  The  hydrogen, 
oxygen,  and  carbon  dioxide  were  removed  as  before.  The  appa- 
ratus, as  shown  in  Figure  III,  was  evacuated  by  means  of  an  oil 
pump.  At  intervals  a small  quantity  of  gas  was  admitted  to  the 
apparatus  to  sweep  out  any  remaining  traces  of  air  that  might 
have  been  present.  When  the  last  of  the  gas  had  been  admitted 

it 

to  the  tube  and  a high  vacuum  obtained,  the  Plucker  tube  was 
sealed  off  from  the  rest  of  the  apparatus. 

Several  attempts  were  made  at  photographing  the  spectrum 
of  the  gas.  This  work  was  done  by  L.  F.  Yntema  in  the  Chemical 
Laboratory  of  the  University  if  Illinois.  The  attempts  along 
this  line  were  in  the  main  part  unsuccessful,  although  suffi- 
cient information  was  obtained  to  establish  the  fact  that  the 
gas  was  air.  In  an  effort  to  obtain  additional  data,  a spectro- 
scopic examination  was  made.  The  spectrum  of  the  gas  as  viewed 
through  the  spectroscope  was  not  very  distinct.  Lines  were 
located,  however,  which  indicated  the  presence  of  argon  and 
bromine.  Two  distinct  band  spectrums  for  nitrogen  were  visible. 
The  presence  of  the  gases  is  easily  explained  in  view  of  the  fact 
that  argon  and  nitrogen  are  present  in  all  air,  and  that  bromine 
is  one  of  the  elements  present  in  the  original  salt  from  which 
the  gas  was  obtained.  Further  spectroscopic  examination  was 
considered  unnecessary,  in  view  of  the  fact  that  all  evidence 
pointed  to  the  conclusion  that  the  gas  was  air. 


* 


. 


' 


. . - . 


. 

. 


* 

. 


. 


11 


hum  ‘“vu  ftmwvj?  V^ed 


:0L  «?|triVNt  j\MMY5l3 


It  is  supposed  that  the  air  was  occluded  during  the  pre- 
cipitation of  the  crystals,  or  else  adsorbed  later.  The  latter 
supposition  is  favored  by  the  fact  that  the  silver  bromate  was 
always  precipitated  from  hot  solutions. 

Analysis  of  residue 

In  order  to  determine  the  composition  of  the  residue 
remaining  in  the  tube  after  the  evolution  of  the  gas,  the  follow- 
ing experiment  was  performed: 


The  residue,  after  being  allowed  to  dry  In  a desiccator 
over  calcium  chloride,  was  weighed.  It  was  then  treated  with 
33$  hydrobromic  acid,  and  evaporated  to  dryness  on  a steam  bath. 
Following  this  evaporation,  the  residue  was  placed  in  an 
electric  drying  oven,  and  heated  for  two  hours  at  a temperature 
of  110  degrees  C.  The  silver  bromate  in  the  residue  was  con- 
verted into  silver  bromide  in  this  process,  the  loss  in  weight 
being  equal  to  the  oxygen  given  off  by  the  silver  bromate.  A 
comparison  of  the  weight  of  the  oxygen  evolved  by  the  residue 
on  treatment  with  hydrobromic  acid,  checks  within  experimental 
error  with  the  theoretical  amount  of  oxygen  that  would  have  been 
evolved  by  an  equal  amount  of  silver  bromate  under  the  same 
conditions,  considering  the  salt  to  be  pure.  This  proves  that 
the  residue  is  nothing  other  than  silver  bromate,  and  that  the 

gas  which  is  evolved  upon  heating  silver  bromate  does  not  result 

from  any  decomposition  of  the  salt* 

Adsorptive  power  of  ignited  Silver  Bromate 

To  obtain  some  information  relative  to  the  adsorptive  power 
of  silver  bromate,  a study  of  the  adsorptive  power  of  a two  gram 
sample  was  made. 

Original  wt.  of  dish  24.2605  grams 

Wt.  of  silver  bromate  2.0000 

Total  wt.  26.2805 

The  dish  containing  the  silver  bromate  was  heated  in  an 
electric  drying  oven  for  ninety-six  hours  at  a temperature  of 


. 


. 


• 

13 

approximately  110  degrees  C.  The  loss  in  weight  of  .0030  grams 
was  considered  to  be  the  weight  of  the  adsorbed  air  driven  off 
from  the  salt.  The  sample  was  now  removed  from  the  §ven  and 
placed  in  a desiccator  over  anhydrous  calcium  chloride.  The 
gained  weight  very  slowly,  and  it  was  assumed  that  this  slow 
increase  in  weight  was  due  to  a slow  adsorption  of  air. 


Wt.  after  heating  for  96  hours 
Wt.  after  standing  for  144  hours 
Wt.  after  standing  for  288  hours 
Wt.  after  standing  for  432  hours 
Original  weight 


26.2775  grams 

26.2780 

26.2785 

26.2793 

26.2805 


This  shows  that  there  is  a slow  adsorption  of  air  which 
will  eventually  continue  until  the  salt  has  as  much  adsorbed 
air  in  it  as  it  originally  had. 


Properties  of  Silver  Bromate 
Behavior  towards  light 

In  studying  the  behavior  of  silver  bromate  towards  light, 

d 

some  pure  silver  bromate  prepared  as  outline  previously  was 
used. 

Pure  silver  bromate  appears  to  be  wholly  unaffected  by 

(2) 

light  when  dry.  The  wet  salt  was  not  found  to  be  so  stable. 

A portion  of  the  pure  salt  was  placed  in  a clean  test  tube, 
covered  with  water,  and  the  top  of  the  tube  sealed  off.  For 
this  work  pure  conductivity  water  was  used.  After  standing 


14 

several  weeks  the  side  next  to  the  window  showed  a slatey  gray 
color,  while  the  rest  was  only  slightly  affected. 

To  see  if  there  was  any  change  in  volume  during  this 
darkening  effect,  due  to  the  formation  of  some  insoluble  gas 
(such  as  oxygen),  some  wet  silver  bromate  was  sealed  in  a tube 
with  a manometer  containing  some  1 nujol'  colored  with  azobenzene 
to  facilitate  reading.  After  standing  for  several  weeks  there 
was  no  displacement  of  the  levels  in  the  arms  as  shown  by  a 
blank  apparatus  containing  only  water  placed  alongside. 

In  a further  study  of  the  stability  of  silver  bromate,  some 
wet  crystals  were  heated.  A marked  darkening  effect  was  noticed. 
This  does  not  occur  with  the  dry  salt  which  is  stable  up  to  its 
melting  point  when  pure. 

This  evidence  leads  to  the  conclusion  that  dry  silver 
bromate  is  stable  towards  light  and  heat,  but  in  the  presence 
of  water  darkens  slowly  at  low  temperatures,  and  rapidly  at 
high  temperatures. 

Decomposition  upon  heating 

On  heating  in  a bath  of  Wood's  metal,  the  purified  silver 
bromate  was  found  to  have  a sharp  melting  point  at  308  to310 
degrees  C.  At  that  temperature  the  salt  melts  to  a clear  color- 

fa) 

less  liquid.  'Stas'  has  reported  that  silver  bromate  when 
heated  to  150  degrees  C.  will  decompose.  This  was  found  to  be 
erroneous  by  experiment  with  100/6  pure  silver  bromate. 


. • , 


. 


. 


\ ...  . . 


. 

15 

After  obtaining  a definite  melting  point  for  silver  bromate, 
a study  of  the  decomposition  was  made.  It  was  found  that  any 
impurity  catalyzes  the  decomposition  of  the  salt  into  a terrific 
explosion.  Using  a sample  of  silver  bromate  weighing  about 
.02  grams,  this  decomposition  was  demonstrated  when  a minute 
quantity  of  ferric  oxide  was  placed  in  the  tube.  Similar  results 
were  obtained  when  a drop  of  'nujol',  a drop  of  water,  or  a 
small  piece  of  dirt  from  the  laboratory  desk  were  placed  in  the 
tube  with  the  silver  salt.  All  these  explosions  took  place  at 
temperatures  ranging  from  260  to  295  degrees  C.  Very  impure 
silver  bromate  can  be  decomposed  explosively  at  temperatures  as 
low  as  155  to  150  degrees. 

The  explosions  resulting  from  several  of  these  decompo- 
sitions were  so  violent  that  the  Wood's  metal  of  the  bath  was 
blown  up  against  the  ceiling.  The  writer  has  found  that  all 
precautions  must  be  taken  for  the  safety  of  the  experimenter 
when  this  decomposition  is  being  studied. 

Upon  decomposition,  silver  bromate  goes  to  pieces  yielding, 
very  probably,  the  following  substances! 

Silver 

Silver  bromide 

Bromine 

Oxygen. 

These  substances  are  probably  the  results  of  the  reaction, 
but  the  statement  cannot  be  verified  because  the  explosive 
nature  of  the  reaction  makes  it  impossible  to  collect  the 

— ■ - — 


. 


, 


.. 


v l * V 


' . 


v. 


• - 

. 


...  V . 


— 

16 

products.  However,  as  a distinct  odor  of  bromine  is  noticed 
after  the  explosion,  it  seems  very  probable  that  silver  bromate 
decomposes  in  some  such  manner  as  outlined  above. 

Existence  of  other  Silver  Oxy-bromine  Compounds 

The  deviation  of  the  composition  of  silver  bromate  from 

100^  purity  and  the  formation  of  a gas  upon  heating,  might  be 

due  to  the  presence  of  small  quantities  of  some  other  silver 

oxy-bromine  compounds,  such  as,  silver  perbromate,  or  silver 

(3) 

hypobromite. 

With  regard  to  the  postulate  of  the  existence  of  silver 

perbromate,  the  following  remarks  should  be  made: 

The  actual  existence  of  perbromic  acid  or  any  of  its  salts 

(4) 

is  very  much  to  be  doubted.  'Kammerer'  reported  that  per- 
bromic acid  was  formed  by  the  action  of  bromine  on  dilute 
perchloric  acid  solutions,  and  that  the  usual  series  of  salts 
were  formed  by  this  acid.  These  results  have  not  been  confirmed 
by  other  investigators. 

The  same  uncertalnity  exists  with  regard  to  the  existence 
of  silver  bromite  and  silver  hypobromite.  With  regard  to  the 
latter  it  should  be  said  that  their  presence  in  silver  bromate 
would  make  the  results  with  the  hydrobromic  acid  analysis  too 
high,  and  not  too  low,  as  was  actually  found.  Furthermore, 
reasoning  from  analogy  with  the  corresponding  silver  oxy-chlorine 
compounds,  they  would  be  more  soluble  than  the  silver  bromate, 
and  their  presence  in  a carefully  purified  substance  like  the 


■ 


. • 


. 1 t 


■ ■ ' 

■ 


17 

salt  used  in  this  work  is  hardly  to  be  expected. 

Nevertheless,  evidence  of  the  presence  of  such  silver- 
oxygen-bromine  compounds  was  sought  in  the  following  way: 

Samples  of  the  purified  silver  bromate,  weighing  one  gram 
each,  were  placed  in  Erlenmeyer  flasks  of  200  cubic  centimeters 
capacity  provided  with  refluxing  condensers.  Ground  glass 
joints  were  used,  so  as  to  eliminate  contact  with  reducing 
materials.  The  salt  was  covered  with  100  cubic  centimeters  of 
distilled  water,  and  the  flasks  placed  in  jackets  designed  to 
heat  them  at  constant  temperatures  for  considerable  lengths  of 
time.  The  heating  was  effected  by  electric  lights,  and  was 
extended  throughout  a period  of  seven  days.  One  lamp  gave  a 
temperature  of  102  to  108  degrees  C.,  while  the  other  gave  85 
degrees.  In  both  flasks,  crusts  of  crystalls  collected  on  the 
surface  of  the  solution.  These  were  skimmed  off,  analyzed  by 
the  hydrobromic  acid  treatment,  and  the  following  data  obtained: 

Wt.  of  dish  silver  bromate  25.3843  grams 

Wt.  of  dish  24.2810 

Wt.  of  silver  bromate  1.1033 

Wt.  of  dish  silver  bromide  residue  25.1687 

Wt.  of  silver  bromide  .8877 

Silver  bromlte  equivalent  1.1146 

This  high  value  was  to  be  expected,  since  the  crust  had  a 
yellow  tint  indicating  the  presence  of  silver  bromide. 

In  the  flask  which  was  heated  to  108  degrees  C.,  a black 


18 

deposit  formed  on  the  bottom.  The  analysis  of  this  deposit 
likewise  indicated  that  partial  decomposition  of  the  silver 
br ornate  had  occurred. 

Confessedly,  this  attempt  to  obtain  evidence  of  a tendency 
of  silver  bromate  to  undergo  decomposition,  forming  products 
intermediate  in  composition,  was  a failure,  as  none  of  the 
products  obtained  appeared  to  be  definite. 

Silver  Bromate  as  a Standard  in  Iodimetry 

On  account  of  the  high  purity,  and  the  ready  method  of 
determining  composition,  the  possibility  of  using  silver  bromate 
as  a standard  in  iodimetry  suggests  itself.  This  was  tried  out 
as  follows i 

The  purity  of  a quantity  of  silver  bromate  was  determined, 
by  converting  it  into  silver  bromide  by  the  hydrobromic  acid 
method  as  outlined  previously  in  this  paper,  and  a value  of 
99*8/£  obtained.  A sample  of  silver  bromate  (1.0463  grams)  was 
weighed  out,  water  was  added,  then  an  excess  of  potassium 
iodide,  and  the  mixture  finally  boiled  for  several  minutes. 

After  cooling,  the  solution  was  made  up  to  250  cubic  centimeters, 
and  25  cubic  centimeter  portions  were  titrated  against  0.1  nor- 
mal sodium  thiosulfate  in  this  manner • 

The  25  cubic  centimeter  sample  was  diluted  to  about  100 
cubic  centimeters,  5 cubic  centimeters  of  dilute  sulfuric  acid 
added,  and  the  liberated  iodine  titrated  in  the  usual  way.  The 
titer  of  the  thiosulfate  was  found  to  be  .09715  normal.  Upon 


. 


, 


* 

19 

standardizing  the  thiosulfate  by  the  usual  arsenous  oxide  method, 
the  titer  was  .09981  normal.  The  work  was  repeated  and  good 
checks  obtained.  The  fact  that  the  arsenous  oxide  method  gave 
definitely  higher  values  than  the  silver  bromate  method  can  be 
explained  only  on  the  assumption  that  the  silver  bromate  had  a 
greater  purity  than  99*8$,  or  that  the  arsenous  oxide  was  not 
100$  pure.  In  order  to  explain  this  discrepancy,  there  are  two 
possibilities : 

(1)  The  silver  bromate  was  purer  than  99*8$, 

(2)  The  arsenous  oxide  was  not  100$  pure,  although  it  was 
of  the  highest  purity  on  the  market  which  is  usually  assumed  to 
be  100$. 

The  work  was  repeated  with  the  100$  pure  silver  bromate 
prepared  above,  giving  almost  identical  results  as  those  obtained 
by  using  the  99.8$  pure  salt.  This  put  the  suspicion  on  the 
arsenous  oxide.  This  was  carefully  purified  by  re crystallization 
from  hydrochloric  acid  solution,  and  sublimation  at  the  lowest 
temperature  possible.  4*95  grams  of  arsenous  oxide  were  weighed 
out  and  made  up  to  one  liter  of  sodium  arsenite  in  the  usual  way, 
giving  a 0.1  normal  arsenite  solution.  This  gave  values  with 
the  sodium  thiosulfate  that  checked  very  acceptably  with  the 
silver  bromate  values.  It  would  appear  then,  that  silver  bromate 
has  two  distinct  advantages  over  the  arsenous  oxide  as  a standard 
in  iodimetry,  viz: 

( 1 ) Higher  purity 

(2)  More  direct  procedure  in  titration. 

By  the  latter  is  meant: 


20 

The  liberated  iodine  from  the  reaction  between  silver  brom- 
ate, potassium  iodide,  and  sulfuric  acid,  is  titrated  directly 
by  the  sodium  thiosulfate;  in  the  arsenous  oxide  method,  the 
arsenite  and  thiosulfate  solutions  are  compared  by  titrating 
against  an  iodine  solution,  involving  two  titrations  in  the  lat- 
ter case  to  one  in  the  former.  This  would  double  the  chance  for 
error.  On  the  other  hand,  the  available  oxygen  in  silver  bromate 
(six  equivalents)  is  high  in  comparison  with  the  reducing  action 
of  sodium  arsenite  (two  equivalents),  so  that  an  error  in  weigh- 
ing out  the  silver  salt  is  slightly  more  significant  in  the  way 
of  error  than  in  the  case  of  arsenous  oxide.  This  work,  however, 
indicates  that  on  the  whole,  the  silver  bromate  should  have  the 
preference  where  maximum  accuracy  is  desired. 

The  definiteness  of  arsenous  oxide  is  brought  into  question 
in  another  way.  Upon  subliming  arsenous  oxide,  partial  decom- 
position occurs: 

2As203 * As4+  302 

Since  arsenic  is  volatile  there  is  a chance  that  the  sublimate 
may  contain  more  or  less  of  the  free  element  which  would  intro- 
duce appreciable  errors  in  the  reducing  value  of  the  oxide. 


«.  ' . 

. 


, 


■ 


*■ 


1.  Analysis  of  silver  bromate,  of  the  highest  purity 
obtainable,  when  converted  into  silver  bromide  by  means  of 
pure  hydrobromic  acid,  indicates  purities  ranging  from  99-6% 
to  100$. 

2*  Upon  heating  in  contact  with  an  inert  medium,  like 
water  or  a saturated  paraffin  oil,  silver  bromate  gives  off  a 
gas,  the  amount  being  approximately  3 to  4 cubic  centimeters 
per  gram  of  the  salt  used. 

3»  Analysis  of  this  gas  shows  it  to  contain  mainly 
oxygen  and  nitrogen,  the  latter  being  something  in  excess  of 
the  amount  in  the  atmosphere.  Spectroscopic  examination  also 
indicates  the  presence  of  an  appreciable  amount  of  argon. 

4.  Silver  bromate,  heated  to  temperatures  below  its 
Ignition  point,  on  cooling,  gradually  increases  in  weight. 

5.  Dry  silver  bromate  of  high  purity  is  stable  toward 
light  and  heat,  but  in  the  presence  of  water  darkens  slowly  at 
low  temperatures,  and  rapidly  at  high  temperatures. 

6.  Silver  bromate  when  pure,  melts  at  308  to  310  degrees 
C.  without  decomposition.  Many  substances  catalyze  the 
decomposition,  which  may  take  place  with  explosive  violence 
even  at  temperatures  as  low  as  150  degrees  C. 

7.  No  evidence  of  the  existance  of  silver  perbromate,  or 
any  other  silver  oxy-bromlne  compounds  was  obtained. 

8.  Silver  bromate  may  be  used  as  a standard  in  iodimetry, 
seeming  to  possess  certain  advantages  over  arsenous  trioxide. 


V BIBLIOGRAPHY 


22 

1.  Abegg  Vol.IIpt.I  pp  712 

2.  Mem.  de  l'acad.  de  Belgique 

3.  Abegg  Vol.II  pt.I  pp  712 

»»  it  it  n ii  « 373 

4.  Jour.  Pract.  Chemie  ( 1863)  £0  190 

Michael  and  Conn  Jour.  Amer.  Chem.Soc.  1901  2£,  89 


